A fuel cell is a device which generates electrical energy by converting chemical energy directly into electrical energy by oxidation of fuel supplied to the cell. Fuel cells are advantageous because they convert chemical energy directly to electrical energy without the necessity of undergoing any intermediate steps, for example, combustion of a hydrocarbon or carbonaceous fuel as takes place in a thermal power station.
A typical fuel cell includes an anode, a cathode and an electrolyte. Fuel and oxidant are supplied to the anode and cathode, respectively. At the anode, the fuel permeates the electrode material and reacts with an anode catalyst layer to form cations (protons) and electrons. The cations migrate through the electrolyte to the cathode. At the cathode, the oxygen-containing gas supply reacts with a cathode catalyst layer to form anions. The electrons produced at the anode travel from the fuel cell anode, through an external load, and back into the cathode of the cell. The anions produced at the cathode react with the cations and electrons to form a reaction product which is removed from the cell.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or pure oxygen) as the oxidant, a catalyzed reaction at the anode produces hydrogen cations from the fuel supply. This type of fuel cell is advantageous because the only reaction product is water. An ion exchange membrane facilitates the migration of hydrogen cations from the anode to the cathode. In addition to conducting hydrogen cations, the membrane isolates the hydrogen fuel stream from the oxidant stream comprising oxygen containing air. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product. The anode and cathode reactions in such fuel cells is shown in the following equations: EQU Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- EQU Cathode reaction: 1/2O.sub.2 +2 H.sup.+ +2.sup.- .fwdarw. H.sub.2 O
A type of fuel cell known as a solid polymer fuel cell ("SPFC") contains a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material. The electrodes are typically formed of carbon fiber paper ("CFP"), and are generally impregnated or coated with a hydrophobic polymer, such as polytetrafluoroethylene. The MEA contains a layer of catalyst at each membrane/electrode interface to induce the desired electrochemical reaction. A finely divided platinum catalyst is typically employed. The MEA is in turn disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and 10 the cathode on the oxidant side. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes.
In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together in series or in parallel to increase the overall power output of the assembly. In such arrangements, the cells are typically connected in series, wherein one side of a given plate serves as an anode plate for one cell and the other side of the plate is the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together by tie rods and end plates. The stack typically includes manifolds and inlets for directing the fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) and the oxidant (substantially pure oxygen or oxygen-containing air) to the anode and cathode flow field channels. The stack also usually includes a manifold and inlet for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust outlets and manifolds for expelling the unreacted fuel and oxidant gases, each carrying entrained water, as well as an outlet manifold for the coolant water exiting the stack.
Conventional fuel cell and stack designs have several inherent disadvantages. First, conventional designs typically employ liquid cooling systems for regulating the cells' operating temperature. Liquid cooling systems are disadvantageous because they require the incorporation of additional components to direct coolant into thermal contact with fuel cells. The power requirements to operate such additional components, such as pumps and cooling fans, represent an additional parasitic load on the system, thereby decreasing the net power derivable from the stack. Such additional components also add volume, weight, complexity and cost to fuel cell designs.
Second, conventional designs employ further parasitic devices such as pumps for the delivery of pressurized fuel and oxidant to the fuel cell. In addition to adding volume, weight, complexity and cost, these parasitic systems also reduce the overall power efficiency of the system.
Third, in conventional stack arrangements it is difficult to identify and replace defective fuel cells without disrupting the operation of the entire fuel cell stack.
The present invention is directed to circumventing one or more of the above-mentioned disadvantages. Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.